(1) Field of the Invention
The present invention is directed to sensors, and, more specifically to a redox hydrogel useful in sensors.
(2) Description of Related Art
In vivo monitoring of glucose is relevant to the management of diabetes. Subcutaneous biosensors can be used to intermittently or continuously monitor the glucose concentration of people in need of such monitoring, particularly people suffering of diabetes. The sensors are also useful in alerting their users and/or medical professionals to hypoglycemia and/or hyperglycemia, and/or impending hypoglycemia and/or hyperglycemia. They are useful in acquiring information about glucose concentration excursions following and/or during events like meals, exercise and sleep, from which an individual's need to take corrective action, such as injecting a particular insulin dose and/or consuming a particular amount of source of glucose at a particular time can be deduced, and are useful in counseling the user to inject the particular insulin dose and/or consume the particular amount of glucose source. Eventually the sensors could become a core component of feedback loops for automatic or manually controlled maintenance of glucose concentrations within a defined range. For example, when used in conjunction with an insulin pump, a specified amount of insulin may be delivered from the pump if the sensor glucose reading is above a set value.
Continuously or intermittently operating glucose sensors, including sensors implanted in the human body, are sought for the management of Type I diabetes. For example, such sensors may provide a warning of imminent or actual hypoglycemia, and hence provide for its avoidance. Hypoglycemia can be fatal, and frequent or continuous monitoring of glucose in diabetic patients is needed in order to ensure that they remain at or near normal blood glucose levels.
Depending on its hydration, polyaniline (PANI) conducts charge carriers by two different mechanisms. When not hydrated, it conducts through one-dimensional bands. Such conduction requires at least one-dimensionally semicrystalline PANI. See Epstein, A., et al. J. MOLECULAR ELECTRONICS 1988, 4, 161-65; Lee, K. et al. NATURE 2006, 441, 65-68. Hydration disrupts the parallel alignment of the chains, and lowers the conduction. When dissolved, PANI behaves as a polymeric redox couple, and if crosslinked and hydrated, it can form an electron-conducting redox hydrogel. See Inzelt, G. J. ELECTROANALYTICAL CHEM. AND INTERFACIAL ELECTROCHEMISTRY 1190, 279, 169-78; Horanyi, G., et al. ELECTROCHIMICA ACTA 1988, 33, 947-52; Brahim, S., et al. MICROCHIMICA ACTA 2003, 143, 123-137. In redox hydrogels, electrons diffuse through electron-transferring collisions between hydrated reducible and oxidizable polymer segments. See Heller, A. CURRENT OPINION IN CHEMICAL BIOLOGY 2006, 10, 664-672. A hydrogel is a matrix that does not dissolve in water, but swells in an aqueous solution, increasing its dry weight by at least a factor of 1.5, i.e., adding at least 50% to its weight or volume when hydrated. Preferably it is a matrix that at least doubles its dry weight when hydrated and most preferably it is a matrix that about triples its dry weight. Hydration lowers the local viscosity and reduces attractive coulombic interactions, thereby increasing segmental mobility and electron diffusion. It also increases the permeability of water-soluble ions and molecules.
Glucose-permeable poly(ethylene glycol)diglycidyl ether (PEGDGE)-crosslinked electron-conducting redox hydrogels of Os2+/3+ complex-comprising polymers with poly(N-vinylimidazole), and partially N-alkylated poly(4-vinylpyridine) backbones have been studied extensively. See Heller, A. CURRENT OPINION IN CHEMICAL BIOLOGY 2006, 10, 664-672. PEGDGE crosslinks primary, secondary, and tertiary amines, as well as heterocyclic nitrogens. When glucose oxidase (GOx) is co-immobilized in some Os2+/3+ complex-comprising hydrogels, it is electrically wired and 3-dimensional glucose electrooxidation catalysts, electrodes at which glucose is electrooxidized at about −0.1 V versus Ag/AgCl and at >1 mA·cm−2 are formed. In contrast, when GOx is physically adsorbed on, entrapped in, or electrodeposited on films of metallic or semiconducting PANI, or otherwise integrated on or in PANI films, the glucose electrooxidation current densities are lower. Such is even the case when PANI is covalently bound to a gold substrate through a flavoenzyme thiol, or through nucleophilic thiol attack at o-positions of PANI quinoimine mers. See, e.g. Grant, E., et al. ELECTROANALYSIS 2006, 18, 26-34; Hua, F., et al. MACROMOLECULES 2003, 36, 9971-78; Mano, N., et al. J. ELECTROANALYTICAL CHEMISTRY 2005, 574, 347-57; Mao, F., et al. J. AM. CHEM. Soc 2003, 125, 4951-57; Chaubey, A., et al. ANALYTICA CHIMICA ACTA 2000, 407, 97-103; Borole. D. D., et al. POLYMERS FOR ADVANCED TECHNOLOGIES 2004, 15, 306-12; Garjonyte, R., et al. BIOSENSORS & BIOELECTRONICS 2000, 15, 445-451; Parente, A. H., et al. APPLIED BIOCHEMISTRY AND BIOTECHNOLOGY 1992, 37, 267-73; Cooper, J. C., et al. ELECTROANALYSIS 1993, 5, 385-97; Hall, E. A., et al. ELECTROANALYSIS 1995, 7, 830-37; Pan, X., et al. SENSORS AND ACTUATORS, B: CHEMICAL 2004, B102, 325-30; Han. C.-C., et al. CHEM. MATER. 1999 11, 480-86; Simon, E., et al. J. ELECTROANALYTICAL CHEMISTRY 2002. 538-539, 253-59.
Alternatively, the doping of PANI with polymer acids has been shown, as well, although not in the presence of GOx, and not co-crosslinked with GOx. See, e.g., Yoo, J. E., et al. J. MATER. CHEM. 2007, 17, 1268-75; Lee, K. S., et al. ADV. FUNC. MATER. 2006, 16, 2409.
Historically, high rate electrocatalytic oxidation of glucose required previously-produced polyaniline-containing microrods, and the use of dissolved, rather then immobilized, GOx. Preparation of the microrods was not a single-step process, and required pyrene sulfonic acid-functionalization of single-walled carbon nanotubes, their embedding in aniline and polystyrene sulfonic acid, electropolymerization of the aniline in porous alumina membranes coated with a conductive gold support, and dissolving the alumina membrane. Although the steady state glucose electrooxidation current densities were not reported for such microrods, voltammetric wave heights of 500 μA cm−2 at 5 mV s−1 scan rate have been observed. See Granot, et al., ELECTROANALYSIS 2006,18, 26-34.
PANI and its adducts with polymer acids, have been known to form, with glucose oxidase, bioelectrocatalysts catalyzing the electrooxidation of glucose; PANI has also been used with glucose oxidase, the glucose oxidase chemically modified with enzyme-penetrating redox couples or enzyme and conductor-bound, e.g. metal or carbon-nanoparticle bound and chemically modified co-factors, to catalyze the electrooxidation of glucose. Because glucose was not as soluble in these compositions as it is in hydrogels, it was mostly or entirely the conductor-contacting surface at which glucose was electrooxidized.
This invention discloses PANI and enzyme comprising hydrogels in which the enzyme's substrate and the product of the enzyme catalyzed reaction, e.g. glucose and gluconolactone, are both soluble. Their greater solubility provides for faster permeation, i.e. in and out diffusion, wherefore the enzyme molecules co-crosslinked in a thicker 3-dimensional water swollen matrix may participate in the current-generating catalytic oxidation or reduction reactions at, for example, an electrode.
Enzyme comprising bioelectrocatalytic redox hydrogels have been made with polymer backbone-bound metal complexes of iron, osmium, ruthenium and nickel cations. These hydrogels comprised, however, fewer redox centers per unit volume than the PANI and enzyme comprising hydrogels of this invention.
U.S. Pat. No. 5,665,222 discloses a biosensor that is stable at 37° C. The biosensor includes a thermostable peroxidase, for example peroxidase isolated from a soybean, which may be crosslinked with a redox polymer to produce a hydrogen peroxide sensor. The biosensor may also comprise additional immobilized enzymes, such as glucose oxidase.
U.S. Pat. Nos. 6,689,265 and 5,972,199 disclose sensors made using redox hydrogels and a thermostable peroxidase.
U.S. Pat. Nos. 6,881,551; 6,514,718; 6,329,161, 6,162,611; 6,121,009; 6,284,478; and 5,593,852 disclose a small diameter flexible electrodes designed for subcutaneous in vivo amperometric monitoring of glucose that may have “one point” in vivo calibration. The sensors may include glucose oxidase electrically wired to a redox polymer.
U.S. Pat. No. 5,356,786 discloses a sensor that comprises a redox polymer.
U.S. Pat. Nos. 6,576,461 and 6,281,006 relate to affinity assays for the detection of a biological ligand and disclose the use of redox polymers in electrical contact with peroxidase to create a catalyst for the electroreduction of hydrogen peroxide.
U.S. Pat. Nos. 7,018,735; 6,531,239; and 6,294,281 disclose redox hydrogels and enzymes used in fuel cells.
The disclosures of all of the above-cited references are incorporated into the present specification in their entirety.